OK, I have consulted with a couple of fellow MVPs and our conclusion is that your document is damaged.

The first step is to recreate the template from which the affected documents have been created. If that template is the blank document template Normal.dotm, then you should exit Word and rename the template. Then restart Word and let it create a new template.

In the new template, recreate your styles. Apply numbering to the built-in headings as discussed in Shauna Kelly's article at http://shaunakelly.com/word/numberin...g20072010.html.

Create a new document based on your new template. Copy the content in the problem document and paste it as plain, unformatted text (for example using the "Text Only" option in the Paste Options button). Manually reformat text by applying the appropriate styles.

__________________
Stefan Blom
Microsoft Word MVP

Microsoft 365 apps for business
Windows 11 Professional